Comparison of groundwater flow under varying hydrologic conditions using MODFLOW

Groundwater flow conditions are variable through time especially under high or low recharge conditions. Streams act similarly under flood or drought conditions. The understanding of these characteristics in a small area located in Decatur, IL is especially important because this is a test site for geologic carbon sequestration. Lake Decatur flows from northeast to southwest in this area and serves as the local, municipal and industrial water source. Therefore, it is essential to know how different hydrologic conditions, including recharge for the area and head values for Lake Decatur, will affect the rate and direction of shallow groundwater flow. The model created compares high recharge and head values and low recharge and head values to the average recharge and head values to determine what changes these conditions will cause in flow rate to Lake Decatur. The model also highlights a groundwater divide and shows how it changes under the conditions discussed previously. In the case that there is a shallow leak of carbon dioxide, these models will help delineate its movement to Lake Decatur. The completed model representing high recharge and head values shows a decrease in gradient moving towards Lake Decatur indicating a decrease in the groundwater flow rate under flood conditions. The model representing low recharge and head values shows an increase in gradient moving towards Lake Decatur indicating an increase in the groundwater flow rate under drought conditions.